The current invention is an light emitting diode (herein referred to as LED) lighting device having multiple features designed to allow the lighting device to maintain luminosity as measured in candelas, over its lifespan. The invention is also designed so that multiple lighting devices can be connected in a network controlled by a host computer, such that the general luminosity over an area (e.g. a tunnel) can be controlled.
Traditionally, roads and tunnel lighting devices have used uses HID (High-intensity Discharge) lamps, powered with high-voltage (e.g. 300VAC to 400VAC).
Existing roadway standards divide the length of the tunnel into a number of regions. Each region requires a lighting intensity (sometimes herein referred to as luminosity) that increases as it is nearer the entrance/exit points (because of the presence of higher illumination from the sun), and decreases towards the middle of the tunnel. As an example, a typical system can use one 130 W lamp per 1.75 m per road lane in order to satisfy the daytime specifications required in the Interior Zone of a tunnel.
A problem with current tunnel lighting lamps is with lifespan. In a typical tunnel lamp which uses one or maybe two illumination sources, the lighting device can easily become unusable should the illumination source break down. In these cases the illumination sources need to be replaced manually before the lighting device can again provide sufficient light. It also is necessary to close lanes in the tunnel while the maintenance is in progress. Thus maintenance of lighting devices can become costly and time consuming.
Therefore, a new lighting device which allows for a longer lifespan without maintenance is desired. The present invention attempts to provide such a lighting device using LEDs as illumination sources. While an individual LEDs is on its own not sufficient to provide lighting for a tunnel, LEDs only cost a fraction of what a typical HID lamp would cost. An LED further uses much less power than that of a typical HID lamp. Therefore, a plurality of LEDs can be provided for a lighting device and the cost and power usage would still be below that of a typical HID lamp. For instance a example LED lighting device could provide lighting for the interior of a tunnel, while using 780 LEDs and being powered by a 24VDC power supply LED lighting devices are known in the art, though most of these lighting devices are use for making billboards and traffic signals. Examples these kinds of lighting devices can be found in U.S. Pat. No. 6,175,342 Nicholson et al., U.S. Pat. No. 6,150,771 Perry, U.S. Pat. No. 6,150,996 Nicholson et al., U.S. Pat. No. 5,514,698 Nicholson et al., U.S. Pat. No. 4,357,671 Miller, U.S. Pat. No. 4,271,408 Teshima et al., and U.S. Pat. No. 4,298,869 Okuno.
Using a plurality of LEDs as a replacement for an standard illumination source is less common, but are for instance described in U.S. Pat. No. 6,255,786 Yen, and U.S. Pat. No. 6,211,626 Lys et al. These lighting devices show that LEDs can be used to replace more standard illumination sources. They do, however, not have features which allow for a constant luminosity to be maintained over an extended lifespan.
Certain patents, such as U.S. Pat. No. 6,236,331 B1 Dussurealt, U.S. Pat. No. 6,153,985 Grossman, and U.S. Pat. No. 5,783,909 Hochstein, propose to compensate for long term LED degradation through a variable current. In these lighting devices, which deal mostly with traffic lights, the luminosity output of traffic lights using LEDs is stabilized by varying the current flow. The lighting devices measure the luminosity output of the LEDs and either increase or decrease the current being supplied to the LEDs as a result. The current control is usually performed either through proportional DC (Direct Current) control, or through PWM (Pulse Width Modulation) of the LED supply. In the context of roadway or tunnel lighting, the use of PWM to control the LED intensity may be problematic. This is because it can lead to visible stroboscopic beat effects in the light superposition of multiple lighting devices each having slightly different, non-synchronized PWM frequencies.
It would therefore be advantageous to have a different way of stabilizing the lighting device's luminosity.